1. Field of the Invention
The present invention relates generally to digital detection apparatus and more particularly to an improved adaptive decision feedback equalizer device for increasing data storage density and decreasing data error rates through processing of the read head output signal.
2. Brief Description of the Prior Art
The advent of the information age brings an enormous demand for the storage of digital data, along with the demands for processing and transmission of such data. The density of information stored in a single system has had to increase to accommodate this growing demand. For each of the past three decades, the capacity of magnetic disk storage units has grown by a factor of 10. This explosive growth has been fueled by several factors: improvements in the design of heads and disks, decreases in the disk-media particle size, decreases in the head gap length and flying height, and improvements in servo accuracy for increased track density. Mark H. Kroyder, "Introduction to the Special Issue on Magnetic Information Storage Technology", Proceedings of the IEEE, November 1986, pp. 1475-1476. More efficient modulation (or "run-length") coding schemes have also been used to increase linear density. K. A. Schouhamer Immink, "Run-Length Limited Sequences", Proceedings of the IEEE, November 1990, pp. 1745-1759.
The growing worldwide demand for digital-storage capacity has also prompted interest in the use of digital signal processing methods as a means of continuing the increases in density. The general similarity of the disk read and write processes to data-detection and transmission in communication has focused a portion of this interest on the application of equalization and coding methods to disk-storage channels. These methods can more efficiently use the available spatial "bandwidth" of disk channels, leading to the desired density increases. In particular, adaptive equalization is attractive since it permits a significant reduction in manufacturing costs by allowing a greater component yield through relaxed tolerances. In addition to providing for increased density, adaptive equalization also permits a reduction in servicing costs because of a reduced need for "fine-tuning" on the customer's premises. Despite the general similarity to a data transmission channel, the data storage channel is significantly different.
In any storage system, there is a channel that consists of the write-head and associated pre-processing circuitry, the storage media itself (usually a magnetic disk or tape, an optical disk or a magneto-optical media), and the read-head and associated data-detection circuitry. These three components are similar to the transmit, channel, and receive functions in digital communication. Thus, communication specialists have been allured, in increasing numbers, by this unusual data channel.
The similarities and differences between the data storage channel and the data transmission channel are illustrated in FIG. 1. In either channel, data is encoded prior to the input and then detected and decoded at the output. Also the goal in both channels is to reliably pass as much information as possible. The storage channel will have high data density, while the transmission channel will have high data rate. A major difference exists between the modulator in the transmission channel and write-processing and write-channel sections in the storage channel. Because of hysteresis effects in the media, only two levels (.+-.1 effectively) can be input to the write channel. Information is stored in the media by the presence or absence of a transition from one data state to another, corresponding to transitions from +1 to -1 (or vice-versa) in the storage channel input. To increase linear density in the storage channel, spacing between transitions must be decreased. In contrast, the transmission channel may use multi-level configurations and carrier modulation to increase the transmitted data rate, because hysteresis is not present in the transmission channel. At high storage densities, severe Intersymbol Interference (ISI) is therefore inevitable; whereas in most transmission channels, ISI is maintained at comparatively moderate levels so that relatively simple equalization is sufficient.
FIG. 1 also indicates that the storage read channel (read-head) processes the following inputs:
1) The data in the storage media,
2) Media noise (including overwrite and data-dependent noises), and
3) Adjacent track interference (correlated, non-Gaussian) on the media.
A final electronics (white-Gaussian) noise is added at the read-channel output. The data transmission channel usually has only an additive noise (white-Gaussian) component, but in some cases, such as subscriber loops, it also suffers from crosstalk and/or adjacent channel (when frequency multiplexed) interference. The adjacent track interference of the storage channel is similar to crosstalk and is of crucial importance in almost all storage systems. In addition to the hysteresis, magnetic media and some read channels exhibit nonlinear effects that become more pronounced as the transition-spacing decreases (density increases). In the transmission channel, these effects are largely absent. The storage channel exhibits random gain fluctuations and actually spectral variations as the position of the head varies with respect to the media. These fluctuations become more pronounced as the "flying height" of the head is reduced, and are a major limiting factor in contact (head "touches" media) recording systems, such as magnetic tapes or floppy disks. These fluctuations are analogous to the flat fades (amplitude drops) that occur in digital radio. Finally, the media thickness varies around the disk. This phenomenon is usually called "once-around modulation", as it is periodic due to continuous variation of media thickness around one track. "Once-around modulation" is similar to a small frequency offset in carrier-modulated data transmission.
For any particular system, both storage and transmission channels are often selected (or switched) from a multitude of similar media. The variation of potentially selected media in efficiently used data transmission channels alone often mandates adaptive detection methods. One would hope that this would not be the case in storage systems, since once a specific head and media are associated during the manufacturing process, then a fixed detection method could be applied. However, this is not the case with interchangeable storage media such as floppy disks or tapes. In addition, for servicing reasons, it is highly undesirable to have a specific storage device "tuned" to a particular head and media, as replacement of either would force this tuning process to be repeated. Furthermore, in fixed hard-disks, the channel changes significantly with changes (as small as 1 .mu. in some disks) in the position of the read-head with respect to the corresponding (earlier) position of the write-head for the same data on the media. This "off-track position" effect is amplified by the previously mentioned "flying height" and "thickness" variations. Thus, adaptive methods are highly desirable in storage-channel detection as well as in transmission-channel detection, even though the sources of channel variation are different. Gains introduced through the use of adaptive equalizers can also be translated into improved (wider) tolerance specifications on heads and media. Thus, manufacturing yield can effectively be improved. Even a small, for example 10%, yield improvement can lead to savings of millions of dollars in manufacturing costs for high-sales-volume storage products.
Adaptive methods are desirable in disk storage channels to mitigate an additional important type of channel variation. This variation occurs with changing radius on the spinning disk. The stored information bits can be construed as successive positions along a track with ones and zeros being distinguished by the presence or absence of transitions in the direction of the magnetic field in each of the positions. Tracks are circular rows of bits arranged concentrically on the disk. Typical high-end hard disks may have 10,000-30,000 b/in. along a track and 1,000-2,000 tracks/in. Thus, the "width" w of a bit is typically smaller than its radial length. The capacity of a disk can be improved by increasing the linear density of bits along the tracks or by increasing the track density through closer placement of adjacent tracks. For the usual case of a disk spinning at a constant angular velocity and constant read data rate 1/T, ISI effects will be most limiting at the Inner Diameter (ID) and least evident at the Outer Diameter (OD). This is because the "transitions" are physically closest together at the ID and farthest apart at the OD. The disk channel's range of radial variation may be large, as the OD/ID ratio may be 1.5 or higher in present storage products, and is likely to increase as smaller disks become more common. A good equalizer characteristic is thus strongly a function of diameter. Adaptive equalization can be used to mitigate the varying amounts of intersymbol interference from ID to OD and thereby improve overall detection (i.e., reduce error probability).
The above uses of adaptive equalization have become increasingly important to future capacity improvements in data storage and retrieval for several reasons. First, the aforementioned potential of significant density increases can satisfy some of the demand for increased storage capacity. Second, the achieved density improvements are not strongly a function of the specific head and media used in the storage channel. This independence renders adaptive equalization compatible with the channel components chosen in almost any storage system, although the adaptive equalizer details can vary significantly from one storage product to another. Even though magnetic disk channels, independent of digital signal processing and coding, can improve through the use of better components (thin film or metallic disks, magneto-optics, magneto-resistive heads, and/or vertical recording, etc.), the adaptive equalizer can provide further density gains. Third, as high-speed digital VLSI (Very Large Scale Integration) circuits become less expensive, digital technologies have the potential for reducing costs in reliably achieving high densities as compared to improving the mechanics of the storage channel. This potential appears imminent as improvement of analog channel components becomes more costly and less effective in terms of potential gains. Finally, the number of advanced component technologies on the horizon for magnetic tape and disk recording has dwindled in recent years, motivating more efficient use, via signal processing and coding, of existing spatial bandwidth on the media. It should be noted that adaptive equalization can potentially be used with optical disks and other magnetic-recording channels. Thus, the objective in using adaptive equalization in disk and tape channels is to increase linear density by continuously varying the receiver detection circuitry to alleviate ISI effects as a function of diameter variation, specific disk channel components, and varying channel mechanics. Of course, the specific use of adaptive equalization may vary considerably among the many different types of media systems sold, just as the specific use of adaptive equalization may vary widely in communication channels (for example, voiceband modems versus digital subscriber loops or digital microwave links).
Crucial to the successful use of adaptive equalization in disk channels is an accurate understanding and characterization of these channels. In a publication entitled "Adaptive Equalization for Magnetic Storage Channels" by J. M. Cioffi et al, IEEE Communications Magazine, pp. 14-29, February 1990, a description is given of the storage channel's important signal-processing characteristics. SNR as a measure for performance evaluation is also discussed. A discussion of the possibility of using decision feedback equalization to increase storage density is given in a paper by K. D. Fisher et al entitled "An Adaptive DFE for Storage Channels Suffering from Nonlinear ISI", Proceedings of 1989 IEEE International Conference on Communications, Boston Mass., June 1989. Copies of these publications are attached hereto as Exhibits A and B and are expressly incorporated herein by reference. Further examples of adaptive equalization techniques are disclosed in: "The Tap-Leakage Algorithm: An Algorithm for the Stable Operation of a Digitally Implemented, Fractionally Spaced Adaptive Equalizer" by R. D. Gitlin et al, The Bell System Technical Journal, vol. 61, no. 8, p. 1817, October 1982; "Adaptive Cancellation of Nonlinear Intersymbol Interference for Voiceband Data Transmission" by Ezio Biglieri et al, IEEE Journal on Selected Areas in Communication, vol. SAC-2, no. 5, pp. 765-777, September 1984; "Adaptive equalization of channel nonlinearities in QAM data transmission systems" by D. D. Falconer, The Bell System Technical Journal, vol. 57, no. 7, pp. 2589-2611, Sept. 1978; "Adaptive cancellation of intersymbol interference for data transmission" by A. Gersho and T. L. Lim, The Bell System Technical Journal, vol. 60, no. 7, pp. 1997-2021, November 1981; "Interference cancellation method and apparatus" by A. Gersho et al, U.S. Pat. No. 4,412,341, Oct. 25, 1983; and "A new digital echo canceller for two wire subscriber lines" by N. Holte and S. Stueflotten, IEEE Trans. Commun., vol. COM-30, pp. 2421-2433, November 1982.